Bipolar transistor with graded base layer

ABSTRACT

A semiconductor material which has a high carbon dopant concentration includes gallium, indium, arsenic and nitrogen. The disclosed semiconductor materials have a low sheet resistivity because of the high carbon dopant concentrations obtained. The material can be the base layer of gallium arsenide-based heterojunction bipolar transistors and can be lattice-matched to gallium arsenide emitter and/or collector layers by controlling concentrations of indium and nitrogen in the base layer. The base layer can have a graded band gap that is formed by changing the flow rates during deposition of III and V additive elements employed to reduce band gap relative to different III-V elements that represent the bulk of the layer. The flow rates of the III and V additive elements maintain an essentially constant doping-mobility product value during deposition and can be regulated to obtain pre-selected base-emitter voltages at junctions within a resulting transistor.

RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/121,444 (now allowed), filed on Apr. 10, 2002, which is acontinuation-in-part of U.S. application Ser. No. 09/995,079, filed onNov. 27, 2001, which claims the benefit of U.S. Provisional ApplicationNo. 60/253,159, filed Nov. 27, 2000, the teachings of both which areincorporated herein in their entirety. U.S. application Ser. No.10/121,444 also claims the benefit of U.S. Provisional Application No.60/370,758, filed Apr. 5, 2002, and of U.S. Provisional Application No.60/371,648, filed Apr. 10, 2002, the teachings of all of which areincorporated herein in their entirety.

GOVERNMENT SUPPORT

The invention was supported in whole or in part by grantF33615-99-C-1510 from the Small Business Technology Transfer (STTR)Program of the U.S. Air Force. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Bipolar junction transistors (BJT) and heterojunction bipolar transistor(HBT) integrated circuits (ICs) have developed into an importanttechnology for a variety of applications, particularly as poweramplifiers for wireless handsets, microwave instrumentation, and highspeed (>10 Gbit/s) circuits for fiber optic communication systems.Future needs are expected to require devices with lower voltageoperation, higher frequency performance, higher power added efficiency,and lower cost production. The turn-on voltage (V_(be,on)) of a BJT orHBT is defined as the base-emitter voltage (V_(be)) required to achievea certain fixed collector current density (J_(c)) The turn-on voltagecan limit the usefulness of devices for low power applications in whichsupply voltages are constrained by battery technology and the powerrequirements of other components.

Unlike BJTs in which the emitter, base and collector are fabricated fromone semiconductor material, HBTs are fabricated from two dissimilarsemiconductor materials in which the emitter semiconductor material hasa large band gap (also referred to as “energy gap”) than thesemiconductor material from which the base is fabricated. This resultsin a superior injection efficiency of carriers from the base tocollector over BJTs because there is a built in barrier impeding carrierinjection from the base back to the emitter. Selecting a base with asmaller band gap decreases the turn-on voltage because an increase inthe injection efficiency of carriers from the base into the collectorincreases the collector current density at a given base-emitter voltage.

HBTs, however, can suffer from the disadvantage of having an abruptdiscontinuity in the band alignment of the semiconductor material at theheterojunction can lead to a conduction band spike at the emitter-baseinterface of the HBT. The effect of this conduction band spike is toblock electron transport out of the base into the collector. Thus,electron stay in the base longer resulting in an increased level ofrecombination and a reduction of collector current gain (β_(dc)). Since,as discussed above, the turn-on voltage of heterojunction bipolartransistors is defined as the base-emitter voltage required to achieve acertain fixed collector current density, reducing the collector currentgain effectively raise the turn-on voltage of the HBT. Consequently,further improvements in the fabrication of semiconductor materials ofHBTs are necessary to lower the turn-on voltage, and thereby improve lowvoltage operation devices.

SUMMARY OF THE INVENTION

The present invention provides an HBT having an n-doped collector, abase formed over the collector and composed of a III-V material thatincludes indium and nitrogen, and an n-doped emitter formed over thebase. The III-V material of the base layer has a carbon dopantconcentration of about 1.5×10¹⁹ cm⁻³ to about 7.0×10^(19 cm) ⁻³. In apreferred embodiment, the base layer includes the elements gallium,indium, arsenic, and nitrogen. The presence of indium and nitrogenreduces the band gap of the material relative to the band gap of GaAs.In addition, the dopant concentration in the material is high, the sheetresistivity (R_(sb)) is low. These factors result in a lower turn-onvoltage relative to HBTs having a GaAs base layer with a similar dopantconcentration.

In a preferred embodiment, the III-V compound material system can berepresented by the formula Ga_(1-x)In_(x)As_(1-y)N_(y). It is known thatthe energy-gap of Ga_(1-x)In_(x)As drops substantially when a smallamount of nitrogen is incorporated into the material. Moreover, becausenitrogen pushes the lattice constant in the opposite direction fromindium, Ga_(1-x)In_(x)As_(1-y)N_(y), alloys can be grown lattice-matchedto GaAs by adding the appropriate ratio of indium to nitrogen to thematerial. Thus, excess strain which results in an increased band gap andmisfit dislocation of the material can be eliminated. The ratio ofindium to nitrogen is thus selected to reduce or eliminate strain. In apreferred embodiment of the present invention, x=3y in theGa_(1-x)In_(x)A_(1-y)N_(y) base layer of the HBT.

In conventional HBTs having a GaAs, the current gain typically decreaseswith increasing temperature as a result of higher injection of holes tothe emitter, higher space charge layer recombination current, andpossible shorter diffusion length in the base. In HBTs having a GaInAsNbase layer, a significant increase in current gain is found withincreasing temperature (approximately 0.3% for each 1° C. rise). Thisresult is interpreted as an increase in diffusion length with increasingtemperature. Such an effect is expected if electrons at the bottom ofthe band are confined in states that are at least partially localized,and with increasing temperature they are thermally excited out of thosestates to others in which the electrons can diffuse more readily. Thus,engineering the base layer with GaInAsN improves temperaturecharacteristics in HBTs of the invention and reduces the need fortemperature compensation sub-circuitry.

HBTs having a GaInAsN base layer have improved common emitter outputcharacteristics over conventional HBTs having a GaAs base layer. Forexample, HBTs having GaInAsN base layers have lower offset and kneevoltages than conventional HBTs having a GaAs base layer.

In one embodiment, the transistor is a double heterojunction bipolartransistor (DHBT) having a base composed of a semiconductor materialwhich is different from the semiconductor material from which theemitter and collector are fabricated. In a preferred embodiment of aDHBT, the Ga_(1-x)In_(x)As_(1-y)N_(y) base layer can be represented bythe formula Ga_(1-x)In_(x)As_(1-y)N_(y), the collector is GaAs and theemitter is selected from InGaP, AlInGaP and AlGaAs.

Another preferred embodiment of the invention relates to a HBT or DHBTin which the height of the conduction band spike is lowered incombination with lowering of the base layer energy gap (E_(gb)).Conduction band spikes are caused by a discontinuity in the conductionband at the base/emitter heterojunction or the base/collectorheterojunction. Reducing the lattice strain by lattice matching the baselayer to the emitter and/or the collector layer reduces the conductionband spike. This is typically done by controlling the concentration ofthe nitrogen and the induim in the base layer. Preferably, the baselayer has the formula Ga_(1-x)In_(x)As_(1-y)N_(y) wherein x is aboutequal to 3y.

In one embodiment, the base can be compositionally graded to produce agraded band gap layer having a smaller band gap at the collector and alarger band gap at the emitter. Preferably, the base layer band gap isabout 20 meV to about 120 meV lower at a surface of the base layer incontact with the collector than at a surface of the base layer incontact with the emitter. More preferably, the band gap of the baselayer varies linearly across the base layer from the collector to theemitter.

Addition of nitrogen and indium to a GaAs semiconductor material lowersthe band gap of the material. Thus, Ga_(1-x)In_(x)As_(1-y)N_(y)semiconductor materials have a lower band gap than that of GaAs. Incompositionally graded Ga_(1-x)In_(x)As_(1-y)N_(y) base layers of theinvention, the reduction in band gap of the base layer is larger at thecollector than at the emitter. However, the average band gap reductionin comparison to the band gap GaAs across the base layer is, typically,about 10 meV to about 300 meV. In one embodiment, the average band gapreduction in comparison to the band gap GaAs across the base layer is,typically, about 80 meV to about 300 meV. In another embodiment, theaverage band gap reduction in comparison to the band gap GaAs across thebase layer is, typically, about 10 meV to about 200 meV. This reducedband gap results in a lower turn-on voltage (V_(be,on)) for HBTs havinga compositionally graded Ga_(1-x)In_(x)As_(1-y)N_(y) base layer than forHBTs having a GaAs base layer because the principal determinant inV_(be,on) is the intrinsic carrier concentration in the base. Theintrinsic carrier concentration (n_(i)) is calculated from the followingformula:n _(i) =N _(c) N _(v) exp (−E _(g) /kT)In the above formula, N_(c) is the effective density of conduction bandstates; N_(v) is the effective density of valence band states; E_(g) isthe band gap; T is the temperature; and k is Boltzmann constant. As canbe seen from the formula, the intrinsic carrier concentration in thebase is largely controlled by the band gap of the material used in thebase.

Grading the band gap of the base layer from a larger band gap at thebase-emitter interface to a smaller band gap at the base-collectorinterface introduces a quasielectric field, which accelerates electronsacross the base layer in npn bipolar transistors. The electric fieldincreases the electron velocity in the base, decreasing the base transittime which improves the RF (radiofrequency) performance and increasesthe collector current gain (also called dc current gain). The dc gain(β_(dc)), in the case of HBTs with heavily doped base layers, is limitedby bulk recombination in the neutral base (n=1). The dc current gain canbe estimated from formula 1:β_(dc) ≅vτ/w _(b)   (1)In formula (1), v is the average minority carrier velocity in the base;τ is the minority carrier lifetime in the base; and w_(b) is the basethickness. Properly grading the base layer in HBTs having a GaInAsN baselayer results in a significant increase in β_(dc) in comparison to anon-graded GaInAsN base layer due to the increased electron velocity.

To achieve a band gap that is graded over the thickness of the baselayer, the base layer is prepared such that it has a higherconcentration of indium and/or nitrogen at a first surface of the baselayer, near the collector, than at a second surface of the base layernearer the emitter. The change in the indium and/or nitrogen contentpreferably changes linearly across the base layer resulting in alinearly graded band gap. Preferably, the concentration of dopant (e.g.,carbon) remains constant throughout the base layer. In one embodiment, aGa_(1-x)In_(x)As_(1-y)N_(y) base layer, for example a base layer of aDHBT, is graded such that x and 3y are about equal to 0.01 at thecollector and are graded to about zero at the emitter. In anotherembodiment, the Ga_(1-x)In_(x)As_(1-y)N_(y) base layer is graded from avalue of x in the range of about 0.2 to about 0.02 at a surface of thebase layer in contact with the collector to a value of x in the range ofabout 0.1 to zero at a surface of the base layer in contact with theemitter, provided that the value of x is larger at the surface of thebase layer in contact with the collector than at the surface of the baselayer in contact with the emitter. In this embodiment, y can remainconstant throughout the base layer or can be linearly graded. When y islinearly graded, the base layer is graded from a value of y in the rangeof about 0.2 to about 0.02 at a surface of the base layer in contactwith the collector to a value of y in the range of about 0.1 to zero ata surface of the base layer in contact with the emitter, provided thatthe value of y is larger at the surface of the base layer in contactwith the collector than at the surface of the base layer in contact withthe emitter. In a preferred embodiment, x is about 0.006 at thecollector and is linearly graded to about 0.01 at the emitter. In a morepreferred embodiment, x is about 0.006 at the collector and is linearlygraded to about 0.01 at the emitter, and y is about 0.001 throughout thebase layer.

In another embodiment, the invention is a method of forming a gradedsemiconductor layer having an essentially linear grade of band gap andan essentially constant doping-mobility product from a first surfacethrough the layer to a second surface. The method includes:

-   -   a) comparing the doping-mobility product of calibration layers,        each of which is formed at distinct flow rates of one of either        an organometallic compound depositing aatom from Group III or V        of the Periodic Table, or of a carbon tetrahalide compound        depositing carbon, whereby the relative organometallic compound        and carbon tetrahalide flow rates required to form an        essentially constant doping-mobility product are determined; and    -   b) flowing the organometallic and carbon tetrahalide compounds        over a surface at said relative rates to form an essentially        constant doping-mobility product, said flow rates changing        during deposition to thereby form an essentially linear grade of        band gap through the graded semiconductor layer.

The base layer can also be dopant-graded such that the dopantconcentration is higher near the collector and decrease gradually acrossthe thickness of the base to the base emitter heterojunction.

Another method of minimizing the conduction band spike is to include oneor more transitional layers at the heterojunction. Transitional layershaving low band gap set back layers, graded band gap layers, dopingspikes or a combination of thereof can be employed to minimize theconduction band spike. In addition, one or more lattice-matched layerscan be present between the base and emitter or base and collector toreduce the lattice strain on the materials at the heterojunction.

The present invention also provides a method of fabricating an HBT and aDHBT. The method includes growing a base layer composed of gallium,indium, arsenic and nitrogen over an n-doped GaAs collector. The baselayer can be grown employing internal and/or external carbon sources toprovide a carbon-doped base layer. An n-doped emitter layer is thengrown over the base layer. The use of an internal and external carbonsource to provide the carbon dopant for the base layer can help form amaterial with a relatively high carbon dopant concentration. Typically,dopant levels of about 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³ are achievedusing the method of the invention. In a preferred embodiment, dopantlevels of about 3.0×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³ can be achievedwith the method of the invention. A higher dopant concentration in amaterial reduces the sheet resistivity and band gap of the material.Thus, the higher the dopant concentration in the base layer of an HBTand DHBT, the lower the turn-on voltage of the device.

The present invention also provides a material represented by theformula Ga_(1-x)In_(x)As_(1-y)N_(y) in which x and y are each,independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹. Preferably, x is aboutequal to 3y. More preferably, x and 3y are about equal to 0.01. In oneembodiment, the material is doped with carbon at a concentration ofabout 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³. In a specific embodiment,the carbon dopant concentration is about 3.0×10¹⁹ cm⁻³ to about 7.0×10¹⁹cm⁻³.

The reduction in turn-on voltage can result in better management of thevoltage budget on both wired and wireless GaAs-based RF circuits, whichare constrained either by standard fixed voltage supplies or by batteryoutput. Lowering the turn-on voltage can also alter the relativemagnitude of the various base current components in a GaAs-based HBT. DCcurrent gain stability as a function of both junction temperature andapplied stress has been previously shown to rely critically on therelative magnitudes of the base current components. A reduction inreverse hole injection enabled by a low turn-on voltage is favorable forboth the temperature stability and long-term reliability of the device.Thus, relatively strain-free Ga_(1-x)In_(x)As_(1-y)N_(y) base materialshaving a high dopant concentration can significantly enhance RFperformance in GaAs-based HBTs and DHBTs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a InGaP/GaInAsN DHBT structure of a preferredembodiment of the invention in which x is about equal to 3y.

FIG. 2 is a Gummel plot which graphical illustrates the base andcollector currents as a function of turn on voltage for an InGaP/GaInAsNDHBT of the invention and for an InGaP/GaAs HBT and a GaAs/GaAs BJT ofthe prior art.

FIG. 3 is a graphical illustration of turn on voltage (at J_(c)=1.78A/cm²) as a function of base sheet resistance for an InGaP/GaInAsN DHBTof the invention and for an InGaP/GaAs HBT and a GaAs/GaAs BJT of theprior art.

FIG. 4 illustrates the photoluminescence spectra measured at 77° K. ofan InGaP/GaInAsN DHBT of the invention and of an InGaP/GaAs HBT of theprior art, both with a nominal base thickness of 1000 Å.Photoluminescence measurements were taken after etching off the InGaAsand GaAs cap layers, selectively stopping at the top of the InGaPemitter. The band gap of the n-type GaAs collector of both theInGaP/GaAs HBT and the InGaP/GaInAsN DHBT was 1.507 eV. The band gap ofthe p-type GaAs base layer of the InGaP/GaAs HBT was 1.455 eV, whereasthe band gap of the p-type GaInAsN base layer of the InGaP/GaInAsN was1.408 eV.

FIG. 5 illustrates double crystal x-ray diffraction (DCXRD) spectra of aInGaP/GaInAsN DHBT of the invention and a InGaP/GaAs HBT of the priorart, both having a nominal base thickness of 1500 Å. The positions ofthe base layers peaks are marked.

FIG. 6 is a Polaron C-V profile which illustrates the carrierconcentration across the thickness of the base layer in an InGaP/GaInAsNDHBT of the invention and an InGaP/GaAs HBT of the prior art. Both theInGaP/GaInAsN DHBT and an InGaP/GaAs HBT have a nominal base thicknessof 1000 Å. Both Polaron profiles are obtained after selectively etchingdown to the top of the base layer.

FIG. 7 a illustrates a preferred InGaP/GaInAsN DHBT structure which hasa transitional layer between the emitter and the base and a transitionallayer and lattice matched layer between the collector and the base.

FIG. 7 b and 7 c illustrates a alternative InGaP/GaInAsN DHBT structurehaving compositionally graded base layers.

FIG. 8 is a graph of the doping*mobility produce as a function of carbontetrabromide flow rate in a carbon doped GaInAsN base layers grown at aconstant indium source gas flow rate (“TMIF” is the trimethyl indiumflow rate).

FIG. 9 is a graph of the TMIF versus the carbon tetrabromide flow rateneeded to obtain a constant doping*mobility product while growing acarbon doped compositionally graded GaInAsN base layers.

FIG. 10 is a graph showing that InGaP/GaInAsN HBTs have a lower turn-onvoltage than InGaP/GaAs HBTs.

FIG. 11 is a graph of the ΔV_(be) versus carbon tetrabromide flow rateof carbon doped GaInAsN base layers grown at a constant TMIF.

FIG. 12 is a graph of the ΔV_(be) versus TMIF.

FIG. 13 is the structure of DHBTs having compositionally graded baselayers used in the experiments in Example 2.

FIG. 14 is the structure of DHBTs having constant composition baselayers used in the experiments in Example 2.

FIG. 15 is a Gummel plot comparing DHBTs having a constant compositionGaInAsN base layer to DHBTs having a compositionally grade GaInAsN baselayer.

FIG. 16 is a graph of the DC current gain as a function of base sheetresistance for DHBTs having a constant composition GaInAsN base layer toDHBTs having a compositionally grade GaInAsN base layer.

FIG. 17 is a Gummel plot comparing a DHBT having a compositionally gradeGaInAsN base layer to two DHBTs having a constant composition GaInAsNbase layer.

FIG. 18 is a graph of comparing DC current gain as a function ofcollector current density for a DHBT having a compositionally gradeGaInAsN base layer to two DHBTs having a constant composition GaInAsNbase layer.

FIG. 19 is a graph comparing the extrapolated current gain cutofffrequency as a function of collector current density of DHBTs having aconstant composition GaInAsN base layer to DHBTs having acompositionally grade GaInAsN base layer.

FIG. 20 is a graph comparing the small signal current gain as a functionof frequency of DHBTs having a constant composition GaInAsN base layerto DHBTs having a compositionally grade GaInAsN base layer.

FIG. 21 is a graph of the peak f_(t) as a function of BV_(cco) ofconstant composition GaInAsN base layer and DHBTs having acompositionally grade GaInAsN base layer to conventional HBTs having aGaAs base layer.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

A III-V material is a semiconductor having a lattice comprising at leastone element from Column III(A) of the Periodic Table and at least oneelement from column V(A) of the Periodic Table. In one embodiment, theIII-V material is a lattice comprised of gallium, indium, arsenic andnitrogen. Preferably, the III-V material can be represented by theformula Ga_(1-x)In_(x)As_(1-y)N_(y) wherein x and y are each,independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹. More preferably, x isabout equal to 3y. In a most preferred embodiment, x and 3y are about0.01.

The term “transitional layer,” as used herein, refers to a layer that isbetween the base/emitter heterojunction or the base/collectorheterojunction and has the function of minimizing the conduction bandspike of the heterojunction. One method of minimizing the conductionband spike is to use a series of transitional layers wherein the bandgaps of the transitional layers gradually decrease from the transitionallayer nearest in proximity to the collector to the transitional layernearest in proximity to the base in a base/collector heterojunction.Likewise, in a emitter/base heterojunction, the band gaps of thetransitional layers gradually decrease from the transitional layernearest in proximity to the emitter to the transitional layer nearest inproximity to the base. Another method of minimizing the conduction bandspike is to use a transitional layer having a graded band gap. The bandgap of a transitional layer can be graded by grading the dopantconcentration of the layer. For example, the dopant concentration of thetransitional layer can be higher near the base layer and can begradually decreased near the collector or the emitter. Alternatively,lattice strain can be used to provide a transitional layer having agraded band gap. For example, the transitional layer can becompositionally graded to minimize the lattice strain at the surface ofthe layer in contact with the base and increase the lattice strain atthe surface in contact with the collector or emitter. Another method ofminimizing the conduction band spike is to use a transitional layerhaving a spike in the dopant concentration. One or more of theabove-described methods for minimizing the conduction band spike can beused in the HBTs of the invention. Suitable transitional layers for theHBTs of the invention include GaAs, InGaAs and InGaAsN.

A lattice-matched layer is a layer which is grown on a material having adifferent lattice constant. The lattice-matched layer typically has athickness of about 500 Å or less and essentially conforms to the latticeconstant of the underlying layer. This results in a band gapintermediate between the band gap of the underlying layer and the bandgap of the lattice-matched material if it were not strained. Methods offorming lattice-matched layers are known to those skilled in the art andcan be found in on pages 303-328 of Ferry, et al., Gallium ArsenideTechnology (1985), Howard W. Sams & Co., Inc. Indianapolis, Ind. theteachings of which are incorporated herein by reference. An example of asuitable material for lattice-matched layers of the HBTs of theinvention is InGaP.

HBTs and DHBTs with Constant-Composition Base Layers

The HBTs and DHBTs of the invention can be prepared using a suitablemetalorganic chemical vapor deposition (MOCVD) epitaxial growth system.Examples of suitable MOCVD epitaxial growth systems are AIXTRON 2400 andAIXTRON 2600 platforms. In the HBTs and the DHBTs prepared by the methodof the invention, typically, an un-doped GaAs buffer layer can be grownafter in-situ oxide desorption. For example, a subcollector layercontaining a high concentration of an n-dopant (e.g., dopantconcentration about 1×10¹⁸ cm⁻³ to about 9×10¹⁸ cm⁻³) can be grown at atemperature of about 700° C. A collector layer with a low concentrationof a n-dopant (e.g., dopant concentration about 5×10¹⁵ cm⁻³ to about5×10¹⁶ cm⁻³) can be grown over the subcollector at a temperature ofabout 700° C. Preferably, the subcollector and the collector are GaAs.The subcollector layer typically has a thickness of about 4000 Å toabout 6000 Å, and the collector typically has a thickness of about 3000Å to about 5000 Å. In one embodiment, the dopant in the subcollectorand/or the collector is silicon.

Optionally, a lattice-match InGaP tunnel layer can be grown over thecollector under typical growth conditions. A lattice-matched layergenerally has a thickness of about 500 Å or less, preferably about 200 Åor less, and has a dopant concentration of about 1×10¹⁶ cm⁻³ to about1×10⁸ cm⁻³.

One or more transitional layers can optionally be grown under typicalgrowth conditions on the lattice-matched layer or on the collector if nolattice-match layer is used. Transitional layer can be prepared fromn-doped GaAs, n-doped InGaAs or n-doped InGaAsN. Transitional layersoptionally can be compositionally or dopant graded or can contain adopant spike. Transitional layers typically have a thickness of about 75Å to about 25 Å. The carbon doped GaInAsN base layer was grown over thecollector if neither a lattice-matched or a transitional layer was used.

The base layer is grown at a temperature below about 750° C. andtypically is about 400 Å to about 1500 Å thick. In a preferredembodiment, the base layer is grown at a temperature of about 500° C. toabout 600° C., Optionally, the carbon doped GaInAsN base layer can begrown over the transitional layer or over the lattice-matched layer if atransitional layer is not used. The base layer can be grown using asuitable gallium source, such as trimethylgallium or triethylgallium, anarsenic source, such as arsine, tributylarsine or trimethylarsine, anindium source, such as trimethylindium, and a nitrogen source, such asammonia or dimethylhydrazine. A low molar ratio of the arsenic source tothe gallium source is preferred. Typically, the molar ratio of thearsenic source to the gallium source is less than about 3.5. Morepreferably, the ratio is about 2.0 to about 3.0. The levels of thenitrogen and indium sources are adjusted to obtain a material which wascomposed of about 0.01% to about 20% indium and about 0.01% to about 20%nitrogen. In a preferred embodiment, the indium content of the baselayer is about three times higher than the nitrogen content. In a morepreferred embodiment, the indium content is about 1% and the nitrogencontent is about 0.3%. In the present invention, a GaInAsN layer havinga high carbon dopant concentration of about 1.5×10¹⁹ cm⁻³ to about7.0×10¹⁹ cm⁻³ can be obtained by using an external carbon sourceorganometallic source, specifically, the gallium source. An example of asuitable external carbon source is carbon tetrabromide. Carbontetrachloride is also an effective external carbon source.

Optionally, one or more transitional layers can be grown of n-dopedGaAs, n-doped InGaAs or n-doped InGaAsN between the base and theemitter. Transitional layers between the base and emitter are relativelylightly doped (e.g., about 5.0×10¹⁵ cm⁻³ to about 5.0×10¹⁶ cm⁻³) andoptionally contain a dopant spike. Preferably, transitional layers areabout 25 Å to about 75 Å thick.

An emitter layer is grown over the base, or optionally over atransitional layer, at a temperature of about 700° C. and is typicallyabout 400 Å to about 1500 Å thick. The emitter layer includes, forexample, InGaP, AlInGaP, or AlGaAs. In a preferred embodiment, theemitter layer includes InGaP. The emitter layer can be n-doped at aconcentration of about 1.0×10¹⁷ cm⁻³ to about 9.0×10¹⁷ cm⁻³. Anemitter-contact layer that includes GaAs containing a high concentrationof an n-dopant (e.g., about 1.0×10¹⁸ cm⁻³ to about 9×10¹⁸ cm⁻³)optionally is grown over the emitter at a temperature of about 700° C.Typically, the emitter contact layer is about 1000 Å to about 2000 Åthick.

A InGaAs layer with a ramped-in indium composition and a highconcentration of an n-dopant (e.g., about 5×10¹⁸ cm⁻³ to about 5×10¹⁹cm⁻³) is grown over the emitter contact layer. This layer typically isabout 400 Å to about 1000 Å thick.

EXAMPLE 1

To illustrate the effect of reducing the band gap of the base layerand/or minimizing the conduction band spike at the emitter/baseheterojunction, three different types of GaAs-based bipolar transistorstructures were compared: GaAs emitter/GaAs base BJTs, InGaP/GaAs HBTs,and InGaP/GaInAsN DHBTs of the invention. A general representation ofInGaP/GaInAsN DHBT structures used in the following experiments isillustrated in FIG. 1. There is only one heterojunction at theemitter/base interface since the base and the collector are both formedfrom GaAs. The GaAs base layer of the InGaP/GaAs HBT has a larger bandgap than the base of the InGaP/GaInAsN DHBT. GaAs/GaAs BJTs have noheterojunctions since the emitter, collector and base are all made ofGaAs. Thus, GaAs BJT structures are used as a reference to determinewhat impact, if any, a conduction band spike at the base-emitterinterface has on the collector current characteristics of InGaP/GaAsHBTs. In the DHBTs of FIG. 1, InGaP is chosen as the emitter materialwith the Ga_(1-x)In_(x)As_(1-y)N_(y) base because InGaP has a wide bandgap, and its conduction band lines up with the conduction band of theGa_(1-x)In_(x)As_(1-y)N_(y) base. Comparison of the InGaP/GaInAsN DHBTsof FIG. 1 and the InGaP. GaAs HBTs are used to determine the effect oncollector current density of having a base layer with a lower band gap.

All of the GaAs devices used in the following discussion haveMOCVD-grown, carbon-doped base layers in which the dopant concentrationvaried from about 1.5×10¹⁹ cm⁻³ to about 6.5×10¹⁹ cm⁻³ and a thicknessvaried from about 500 Å to about 1500 Å, resulting in a base sheetresistivity (R_(sb)) of between 100 Ω/□ and 400 Ω/□. Large area devices(L=75 μm×75 μm) were fabricated using a simple wet-etching process andtested in the common base configuration. Relatively small amounts ofindium (x˜1%) and nitrogen (y˜0.3%) were added incrementally to form twoseparate sets of InGaP/GaInAsN DHBTs. For each set, growth has beenoptimized to maintain high, uniform carbon dopant levels (>2.5×10¹⁹cm⁻³), good mobility (˜85 cm²/V-s), and high dc current gain (>60 atR_(sb)˜300 Ω/□).

Typical Gummel plots from a GaAs/GaAs BJT, an InGaP/GaAs HBT and anInGaP/GaInAsN DHBT with comparable base sheet resistivities were plottedand overlaid in FIG. 2. The collector currents of the InGaP/GaAs HBT andGaAs/GaAs BJT were indistinguishable for over five orders of magnitude(decades) of current until differences in effective series resistanceimpacted the current-voltage characteristics. On the other hand, thecollector current of an InGaP/GaInAsN DHBT was two-fold higher than thecollector current of the GaAs/GaAs BJT and the InGaP/GaAs HBT over awide bias range, corresponding to a 25.0 mV reduction in turn-on voltageat a collector current density (J_(c)) of 1.78 A/cm². The observedincrease in the low-bias base current (n=2 component) in the BJT isconsistent with an energy-gap driven increase in space chargerecombination. The neutral base recombination component of the basecurrent in the InGaP/GaInAsN DHBT was driven higher than in theInGaP/GaAs HBT because of the increase in collector current, as well asreduction in the minority carrier lifetime or an increase in the carriervelocity (I_(nbr)=I_(c)w_(b)/vr). InGaP/GaInAsN DHBT devices preparedto-date have achieved a peak dc current gain of 68 for a device having abase sheet resistivity of 234 Ω/□, corresponding to a decrease inturn-on voltage of 11.5 mV, and a peak dc current gain of 66 for adevice having a base sheet resistivity of 303 Ω/□, corresponding to adecrease in turn-on voltage of 25.0 mV. This represents the highestknown gain-to-base-sheet-resistance ratios (β/R_(sb)˜0.2-0.3) for thesetypes of structures. The energy-gap reduction in theGa_(1-x)In_(x)As_(1-y)N_(y) base, is responsible for the observeddecrease in turn-on voltage, as demonstrated by low temperature (77° K.)photoluminescence. DCXRD measurements indicate the lattice mismatch ofthe base layer is minimal (<250 arcsec).

In the diffusive limit, the ideal collector current density of a bipolartransistor as a function of base-emitter voltage (V_(be)) can beapproximated as:J _(c)=(qD _(n) n ² _(ib) /p _(b) w _(b)) exp (qV _(be) /kT)   (2)where

-   -   p_(b) and w_(b) base doping and width;    -   D_(n) diffusion coefficient;    -   n_(ib) intrinsic carrier concentration in the base.        By expressing n_(ib) as a function of base layer energy-gap        (E_(gb)) and rewriting the product of base doping and thickness        in terms of base sheet resistivity (R_(sb)), the turn-on voltage        can be expressed as a logarithmic function of base sheet        resistance        V_(be)=−A In [R_(sb)]+V_(o)   (3)        with        A=(kT/q)   (4)        and        V_(o)=E_(gb) /q−(kT/q)In[q ²μN_(c)N_(v) D _(n) /J _(c)]  (5)        where N_(c) and N_(v) are the effective density of states in the        conduction and valence bands and μ is the majority carrier        mobility in the base layer.

FIG. 3 plots the turn-on voltage at J_(c)=1.78 A/cm² as a function ofbase sheet resistivity for a number of InGaP/GaAs HBTs, GaAs/GaAs BJTs,and InGaP/GaInAsN DHBTs. The turn-on voltage of both the InGaP/GaAs HBTsand the GaAs/GaAs BJTs, which do not have any conduction band spike,qualitatively exhibit the same logarithmic dependence on base sheetresistivity expected from equation (2). Quantitatively, the variation ofbase-emitter voltage (V_(be)) with base sheet resistivity is less severethan represented by equation (3) (A=0.0174 instead of 0.0252 mV).However, this observed reduction in A is consistent with thequasiballistic transport through thin base GaAs bipolar devices.

Comparison with the characteristics of GaAs/GaAs BJTs leads to theconclusion that the effective height of the conduction band spikeInGaP/GaAs HBTs can be zero, with the collector current exhibiting ideal(n=1) behavior. Thus, InGaP/GaAs HBTs can be engineered to haveessentially no conduction band spike. Similar results were found byprevious work for AlGaAs/GaAs HBTs. To further lower the turn-on voltagefor these devices for a fixed base sheet resistivity requires the use ofa base material with a lower energy gap but which still maintains theconduction band continuity. Ga_(1-x)In_(x)As_(1-y)N_(y) can be used toreduce E_(gb) while maintaining near lattice-matching conditions. Asseen in FIG. 3, the turn-on voltage of two sets of InGaP/GaInAsN DHBTsfollows a logarithmic dependence on base sheet resistivity indicatingthat the conduction band spike is about zero. In addition, the turn-onvoltage is shifted downward by 11.5 mV in one set and by 25.0 mV in theother set (dashed lines) from that observed for InGaP/GaAs HBTs andGaAs/GaAs BJTs.

The above experiment shows that the turn-on voltage of GaAs-based HBTscan be reduced below that of GaAs BJTs by using a InGaP/GaInAsN DHBTstructure. A low turn-on voltage is achieved through two key steps. Thebase-emitter interface is first optimized to suppress the conductionband spike by selecting base and emitter semiconductor materials inwhich the conduction bands are at about the same energy level. This issuccessfully done using InGaP or AlGaAs as the emitter material and GaAsas the base. A further reduction in turn-on voltage was thenaccomplished by lowering the band gap of the base layer. This wasachieved while still maintaining lattice matching throughout the entireHBT structure by adding both indium and nitrogen to the base layer. Withproper growth parameters, a two-fold increase in collector currentdensity was achieved without significantly sacrificing base doping orminority carrier lifetime (p=68 at R_(sb)=234 Ω/□). These resultsindicate that the use of a Ga_(1-x)In_(x)As_(1-y)N_(y) material providesa method for lowering the turn-on voltage in GaAs-based HBTs and DHBTs.Since incorporation of indium and nitrogen in GaAs lowers the band gapof the material, larger reductions in turn-on voltage within GaAs basedHBTs and DHBTs are expected as a larger percentage of indium andnitrogen is incorporated into the base if a high p-type dopingconcentration is maintained.

The energy-gap reduction in the GaInAsN base, assumed to be responsiblefor the observed decrease in turn-on voltage, has been confirmed by lowtemperature (77° K.) photoluminescence. FIG. 4 comparesphotoluminescence spectra from an InGaP/GaInAsN DHBT and a conventionalInGaP/GaAs HBT. The base layer signal from the InGaP/GaAs HBT is at alower energy than the collector (1.455 eV vs. 1.507 eV) because ofband-gap-narrowing effects associated with high-doping-levels. The baselayer signal from the InGaP/GaInAsN DHBT which appears at 1.408 eV isreduced because of band-gap-narrowing effects and a reduction in thebase layer energy gap caused by incorporation of indium and nitrogen inthe base layer. In this comparison, the doping levels are comparable,suggesting the 47 meV reduction in the position of the base layer signalcan be equated to a reduction in the base layer energy gap in theGaInAsN base as compared with the energy gap of the GaAs base. Thisshift in photoluminescence signal correlates very well with the measured45 mV reduction in turn-on voltage. In the absence of a conduction bandspike, the turn-on voltage reduction can be directly related to thedecrease in base layer energy gap.

The DCRXD spectra shown in FIG. 5 illustrates the effect of addition ofcarbon dopants and indium to a GaAs semiconductor. FIG. 5 shows theDCRXD spectra from both an InGaP/GaInAsN DHBT and a standard InGaP/GaAsHBT of comparable base thickness. In the InGaP/GaAs HBT, the base layeris seen as a shoulder on the right hand side of the GaAs substrate peak,approximately corresponding to a position of +90 arcsecs, due to thetensile strain generated from the high carbon dopant concentration of4×10¹⁹ cm⁻³. With the addition of indium, the base layer peak is at −425arcsec in this particular InGaP/GaInAsN DHBT structure. In general, theposition of the peak associated with the GaInAsN base is a function ofthe indium, nitrogen, and carbon concentrations. The addition of indiumto GaAs adds a compressive strain, while both carbon and nitrogencompensate with a tensile strain.

Maintaining high p-type doping levels as indium (and nitrogen) are addedto carbon doped GaAs requires careful growth optimization. A roughestimate of the active doping level can be obtained from a combinationof measured base sheet resistivity and base thickness values. The basedoping can also be confirmed by first selectively etching to the top ofthe base layer and then obtaining a Polaron C-V profile. FIG. 6 comparessuch Polaron C-V doping profiles from a GaAs base layer and a GaInAsNbase layer. In both case, doping levels exceeded 3×10¹⁹ cm⁻³.

FIG. 7 a shows an alternative structure for DHBTs having constantcomposition GaInAsN base layer (10) that employs transitional layers (20and 30) between the emitter/base and the collector/base junction. Inaddition, a lattice-match InGaP tunnel layer (40) is employed betweenthe transitional layer and the collector.

DHBTs with Compositionally-Graded Base Layers

All layers in DHBTs having a compositionally-graded base layer can begrown in a similar fashion as DHBTs having a base with a constantcomposition except the base layer as a graded band gap firm one junctionthrough the layer to another junction of the transistor. For example, acarbon-doped and bond gap-graded GaInAsN base layer can be grown overthe collector if neither a lattice-matched nor a transitional layer isused. Optionally, the carbon doped graded GaInAsN base layer can begrown over the transitional layer or over the lattice-matched layer if atransitional layer was not used. The base layer can be grown at atemperature below about 750° C. and typically is about 400 Å to about1500 Å thick. In one embodiment, the base layer is grown at atemperature of about 500° C. to about 600° C. The base layer can begrown using a gallium source, such as, for example, trimethylgallium ortriethylgallium, an arsenic source, such as arsine, tri(t-butyl)arsineor trimethylarsine, an indium source, such as trimethylindium, and anitrogen source, such as ammonia, dimethylhydrazine or t-butylamine. Alow molar ratio of the arsenic source to the gallium source ispreferred. Typically, the ratio molar ratio of the arsenic source to thegallium source is less than about 3.5. More preferably, the ratio isabout 2.0 to about 3.0. The levels of the nitrogen and indium sourcescan be adjusted to obtain a material in which the content of the GroupIII element is about 0.01% to about 20% indium and the content of theGroup V element is about 0.01% to about 20% nitrogen. In a specificembodiment, the content of the Group III element that is indium isvaried from about 10% to 20% at the base-collector junction to about0.01% to 5% at the base-emitter junction and the content of the Group Velement that is nitrogen essentially is constant at about 0.3%. Inanother embodiment, the nitrogen content of the base layer is aboutthree times lower than the indium content. As discussed above in regardto GaInAsN base layers having a constant composition, it is believedthat a GaInAsN layer having a high carbon dopant concentration, of about1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³, can be achieved by using anexternal carbon source, such as a carbon tetrahalide, in addition to thegallium source. The external carbon source used can be, for example,carbon tetrabromide. Carbon tetrachloride is also an effective externalcarbon source.

Since organoindium compounds that are used as the indium source gascontribute a different amount of carbon dopant to the GaInAsN base layerthan the organogallium compounds that are used as the gallium sourcegas, the carbon dopant source gas flow typically is adjusted during thegrowth of the base layer so as to maintain a constant carbon dopingconcentration in the compositionally-graded GaInAsN base layer. In oneembodiment, change in the carbon source gas flow over thecompositionally graded base layer is determined using the methoddescribed below.

Carbon and Trimethylindium Source Flow Rate Calibration Procedure forGraded GaInAsN and/or Graded InGaAs Semiconductor Layers

At least two sets of calibration HBTs are prepared, in which each setcontains at least two members (DHBTs can be used instead of HBTs). Thebase layer thickness is ideally the same for all calibration HBTs formedbut is not a requirement, and each HBTs has a constant composition, suchas a constant composition of GaInAsN or GaInAs base layer and a constantcarbon dopant concentration throughout the layer. Each set is grown at adifferent Group III or Group V additive (such as indium for Group III ornitrogen for Group V) source gas flow rate than another set so that themembers of each set have a different gallium, indium, arsenic andnitrogen composition than that of members of a different set. By way ofexample, indium will be employed as the additive which affects band gapgradation. Each member of a particular set is grown at a differentexternal carbon source (e.g., carbon tetrabromide, or carbontetrachloride) flow rate so that each member of a particular set has adifferent carbon dopant level. The doping*mobility product is determinedfor each member and graphed against the carbon source flow rate. Thedoping*mobility product varies proportionately with the carbon sourcegas flow rate for the members of each set. Doping*mobility product vs.carbon tetrabromide flow rate for five sets of HBTs is graphed in FIG.8. Alternatively, each set of calibration HBTs could be formed bymaintaining a constant flow rate of the carbon source gas, such ascarbon tetrabromide, and each separate sample in each set could beformed at a distinct Group m or Group V additive flow rate relative tothe flow rate of the other sources gases.

The flow rate of carbon source gas versus indium source gas needed toobtain a constant doping*mobility product is obtained by drawing a lineacross the graph in FIG. 8 at a constant doping*mobility product (e.g.,a line parallel to the x-axis). Where this line intersects with thestraight lines of each set represents the external carbon source flowneeded to obtain this doping*mobility product value when the indiumsource gas flow is set at the flow rate for that set. The externalcarbon source gas flow rate versus indium source gas flow rate for oneconstant doping*mobility product value is graphed in FIG. 9. Similarlines for different doping*mobility products can be graphed in the sameway.

The collector current of each HBT is graphed as a function ofbase-emitter voltage (V_(be)) and the curve obtained is compared to agraph of an HBT that has a GaAs base layer but otherwise is identical tothe member in the set to which it is compared (e.g., has the same dopantconcentration, the same thickness of base, emitter and collector layers,ect.). The voltage difference between the curves at a particularcollector current is the change in the base emitter voltage, V_(be)(ΔV_(be)), attributed to the lower energy gap of the base layer causedby addition of indium and nitrogen during formation of the base layer.FIG. 10 shows a plot of the collector current as a function of V_(be)for an HBT having a GaInAsN base layer and an HBT having a base layer ofGaAs. The horizontal arrow drawn between the two curves is the ΔV_(be).The ΔV_(be) for each member of all the sets is determined and plottedvs. carbon source gas flow. The ΔV_(be) vs. carbon tetrabromide flow foreach member of the five sets of HBTs used to form the graph in FIG. 8were plotted in FIG. 11. Note that the ΔV_(be) for the members of a setspan a range of ΔV_(be) for that set to which a straight line can befitted. These lines are then used to determin (interpolate) the ΔV_(be)values for HBTs that could be grown using the same indium source gasflow rate of a particular set but with carbon source gas flow rates thatare different from the other members of the set.

The ΔV_(be) for a constant doping*mobility product varies linearly as afunction of indium source gas flow rate, as can be seen when theinterpolated ΔV_(be) for a constant doping*mobility product is plottedas a function of indium source gas flow rate. FIG. 12 shows this plotfor the five sets used in FIG. 11.

The graph shown in FIG. 12 is used to determine the indium source gasflow needed to obtain the desired ΔV_(be) at the base-emitter andbase/collector junctions. Once the indium source gas flow is determined,FIG. 9 is used to determine the carbon source gas flow needed at thatindium source gas flow to obtain the desired dopant*mobility product.The same procedure is followed to determine the desired indium sourcegas flow and carbon source gas flow at the base-collector junction tomaintain the desired constant dopant*mobility product in thecompositionally graded GaInAs or GaInAsN layer. The indium source gasflow and carbon source gas flow are varied linearly relative to thegallium and arsenic levels when the base layer is grown from thebase-collector junction to the base-emitter junction to the valuesdetermined for these source gases at these junctions to obtain alinearly graded base layer having a desired band gap grade.

EXAMPLE 2

All of the GaAs devices used in the following discussion wereMOCVD-grown, carbon-doped base layers in which the dopant concentrationvaried from about 3.0×10¹⁹ cm⁻³ to about 5.0×10¹⁹ cm⁻³ and a thicknesswhich varied from about 500 Å to about 1500 Å, resulting in a base sheetresistivity (R_(sb)) of between 100 Ω/□ and 650 Ω/□. Large area devices(L=75 μm×75 μm) were fabricated using a simple wet-etching process andtested in the common base configuration. Relatively small amounts ofindium (x˜1% to 6%) and nitrogen (y˜0.3%) were added incrementally toform two separate sets of InGaP/GaInAsN DHBTs. For each set, growth wasoptimized to maintain relatively high, uniform carbon dopant levels(>2.5×10¹⁹ cm⁻³), good mobility (˜85 cm² V-s), and high dc current gain(>60 at R_(sb)˜300 Ω/square). The structure of a DHBT used in thefollowing experiments having a compositionally graded GaInAsN base layeris shown in FIG. 13. Alternative structures for DHBTs havingcompositionally garded base layers is shown in FIGS. 7 b and 7 c. Thestructure of a DHBT having a constant composition GaInAsN base layerused in the following experiments for comparison is shown in FIG. 14.

FIG. 15 shows Gummel plots from a constant and a graded base DHBT withcomparable turn-on voltages and base sheet resistance. The neutral basecomponent of the base current is significantly lower in the graded basestructure, which exhibits a peak dc current gain which can be over afactor of 2 higher than constant base structures. FIG. 16 compares dccurrent gain as a function of base sheet resistance from similarconstant and graded DHBT structures with varying thicknesses. Theincrease in gain-to-base-sheet-resistance ratio is readily apparent.While the gain-to-base-sheet-resistance ratio of DHBTs depends on thegrowth conditions utilized and the specific details of the overallstructure, a consistent 50 % to 100% increase in dc current gain inDHBTs with a graded base layer over DHBTs with a constant base layer hasbeen observed.

FIG. 17 and 18 compare the Gummel plots and gain curves from a gradedbase structure to two constant base structures. The base composition ofthe first constant base structure corresponding to the composition ofbase layer of the graded base at base-emitter junction. The basecomposition of the second constant base structure corresponding to thecomposition of base layer of the graded base at base-emitter junction.The turn-on voltage of the graded base structure is intermediate betweenthe tow endpoint structures, but is weighted towards the base-emitterendpoint. The dc current gain of the graded bas structure is between 50%and 95% higher than the endpoint structures, indicating most of theincrease in dc current gain results from an increased electron velocity.

On-wafer FF testing was performed using an HP8510C parametric analyzeron 2 finger, 4 μm×4 μm emitter area devices. Pad parasitic werede-embedded using open and short structures, and the current gain cutofffrequency (f_(t)) was extrapolated using a −20 dB/dicade slope of thesmall signal current gain (H21). FIG. 19 summarizes the f_(t) dependencewith collector current density (J_(c)) on both structures. FIG. 20illustrates the small signal gain versus frequency at one particularbias point. As J_(c) increases and the base transit time (t_(b)) beginsto play a limiting role in the total transit time, the f_(t) of thegraded base structure becomes notably larger than the constantcomposition structure, despite the greater base thickness of the gradedbase structure (constant base layer is 60 nm thick whereas graded baselayer is 80 nm thick). The peak f_(t) of the 60 nm constant compositionGaInAsN base is 53 GHz, while the 80 nm compositionally graded GaInAsNbase has a peak f_(t) of 60 GHz. Thus, the current gain cutoff frequencyis increased by 13%.

To better compare the RF results of DHBTs having a constant and a gradedGaInAsN base layer to one another and to conventional GaAs HBTs, thef_(t) values form FIG. 19 were plotted as a function of thezero-input-current breakdown voltage (BV_(ceo)) that can be applied tothe transistor. This plot was compared to the peak or near peak f_(t)values of conventional GaAs HBTs quoted in the literature. A fairly widedistribution in the f_(t) values of the conventional GaAs HBTs wasexpected, as this data was compiled from many groups using differentepitaxial structures, device sizes, and test conditions, and is onlymeant to give a sense of current industry standards. The BV_(ceo) mostoften had to be estimated from quoted collector thickness assuming therelations between the collector thickness (X_(c)), BV_(cbo) and BV_(ceo)shown in FIG. 21. Also shown in FIG. 21 are three simple calculations ofthe expected dependence of f_(t) on BV_(ceo) assuming the transit timethrough the space charge layer of collector (τ_(sclc)) is simply relatedto X_(c) via the electron saturation (drift) velocity (v_(s)). In thebaseline calculation, the τ_(b) is assumed to be 1.115 ps, as expectedfrom Monte-Carlo calculations for a 1000 Å GaAs base layer, and the sumof the remaining emitter and collector transit times (τ_(e)+τ_(c)) wastaken as 0.95 ps.

Examination of FIG. 21 indicates that while the f_(t) of the constantcomposition GaInAsN is not entirely outside the range expected forconventional GaAs-based HBTs, it is clearly on the low end of thedistribution. The graded base structure is notably improve. The secondcalculation (baseline with τ_(b) reduced by ⅔) suggests the base transittime was decreased by approximately 50% relative to the constantcomposition structure. This indicate that a 2× increase in velocity ofcarriers was achieved in the graded base layer as compared to theconstant composition base layer since a 2× increase in velocity combinedwith a 33% increase in base thickness is expected to lead to a reductionin τ_(b) of ½×{fraction (4/3)}=⅔. The third calculation (baseline withτ_(b) reduced by ⅓ and (τ_(e)+τ_(r)) by ½) approximates situations inwhich thin and/or graded base structures are employed along withimproved device layout and size (to minimize τ_(b), τ_(e) and τ_(c)).

Equivalents

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method of fabricating a heterojunction bipolar transistor,comprising the steps of: a) growing a base layer comprising gallium,indium, arsenic and nitrogen over an n-doped GaAs collector layer from agallium, indium, arsenic, and nitrogen source, wherein the base layer isp-doped with carbon from an external carbon source; and b) growing ann-doped emitter layer over the base layer.
 2. The method of claim 1,wherein the external carbon source is carbon tetrabromide or carbontetrachloride.
 3. The method of claim 2, wherein the gallium source isselected from trimethylgallium and triethylgallium.
 4. The method ofclaim 3, wherein the nitrogen source is ammonia, dimethylhydrazine ortertiarybutylamine.
 5. The method of claim 4, wherein the ratio of thearsenic source to the gallium source is about 2.0 to about 3.5.
 6. Themethod of claim 5, wherein the base is grown at a temperature of lessthan about 750° C.
 7. The method of claim 6, wherein the base is grownat a temperature of about 500° C. to about 600° C.
 8. The method ofclaim 6, wherein the base layer comprises a layer of the formulaGa_(1-x)In_(x)As_(1-y)N_(y), wherein x and y are each, independently,about 1.0×10⁻⁴ to about 2.0×10⁻¹.
 9. The method of claim 8, wherein x isabout equal to 3y.
 10. The method of claim 8, wherein the collectorincludes GaAs and the emitter includes a material selected from thegroup consisting of InGaP, AlInGaP, and AlGaAs, and wherein thetransistor is a double heterojunction bipolar transistor.
 11. The methodof claim 8, further comprising the step of growing an n-doped firsttransitional layer over the collector layer prior to growing the baselayer, and wherein the base layer is grown over the n-doped firsttransition layer, and wherein the first transitional layer has a gradedband gap or a band gap that is smaller than the band gap of thecollector.
 12. The method of claim 11, wherein the first transitionallayer is selected from the group consisting of GaAs, InGaAs, andInGaAsN.
 13. The method of claim 12, further comprising the step ofgrowing a second transitional layer over the base prior to growing then-doped emitter layer, wherein the second transitional layer has a firstsurface contiguous with a surface of a first surface of the base and asecond surface contiguous with a surface of the emitter, and wherein thesecond transitional layer has a doping concentration at least one orderof magnitude less than the doping concentration of the emitter.
 14. Themethod of claim 13, wherein the second transitional layer is selectedfrom the group consisting of GaAs, InGaAs, and InGaAsN.
 15. The methodof claim 14, wherein the first transitional layer, the secondtransitional layer, or both the first and the second transitional layerformed have a doping spike.
 16. The method of claim 14, furthercomprising the step of growing a latticed matched layer over thecollector prior to growing the n-doped first transitional layer, whereinthe lattice matched layer has a first surface contiguous with a firstsurface of the collector and a second surface contiguous with a secondsurface of the first transitional layer.
 17. The method of claim 16,wherein the lattice matched layer includes InGaP.
 18. A method offorming a graded semiconductor layer having an essentially linear gradeof band gap and an essentially constant doping-mobility product, from afirst surface through the layer to a second surface, comprising thesteps of: a) comparing the doping-mobility product of calibrationlayers, each of which is formed at a distinct flow rate of one of eitheran organometallic compound depositing an atom from Group III or V of thePeriodic Table, or of a carbon tetrahalide compound depositing carbon,whereby the relative organometallic compound and carbon tetrahalide flowrates required to form an essentially constant doping-mobility productare determined; and b) flowing the organometallic and carbon tetrahalidecompounds over a surface at said relative rates to form an essentiallyconstant doping-mobility product, said flow rates changing duringdeposition to thereby form an essentially linear grade of band gapthrough the graded semiconductor layer.
 19. The method of claim 18,further including the step of depositing the graded layer on a secondsemiconductor layer during fabrication of a junction device.
 20. Themethod of claim 19, wherein the second semiconductor layer is acollector layer.
 21. The method of claim 19, wherein the secondsemiconductor layer is an emitter layer.
 22. The method of claim 18,wherein the graded semiconductor layer includes gallium, indium andarsenic, and wherein the organometallic compound that determines therate of deposit of the carbon tetrahalide to form an essentiallyconstant doping-mobility product includes an organo-indium compound. 23.The method of claim 22, wherein the carbon tetrahalide is CBr₄.
 24. Themethod of claim 23, wherein the organometallic compound further includesa nitrogen source gas.
 25. The method of claim 24, wherein the secondsemiconductor layer in which the graded semiconductor layer is depositedincludes GaAs.
 26. The method of claim 25, further including the step ofdepositing a third semiconductor layer on the base layer.
 27. The methodof claim 26, wherein the third semiconductor layer is InGaP.
 28. Themethod of claim 27, wherein the doping-mobility product for eachcalibration layer is related to a bandgap, whereby bandgaps at the firstand second surfaces of a graded layer, in combination with adoping-mobility product, will be calibrated to relative rates oforganometallic and carbon tetrahalide flow rates required for depositionof said graded semiconductor layer.
 29. The method of claim 28, whereinsaid bandgaps are calibrated as base-emitter voltages of junctiondevices employing said calibration layers as base layers, relative toGaAs.
 30. The method of claim 29, wherein the graded semiconductor baselayer formed is a base layer in a heterojunction bipolar transistor. 31.The method of claim 30, wherein the flow rates of the organometallic andcarbon tetrahalide cause the bandgap of the resultant graded base layerto decrease from a base-emitter junction to a base-collector junction ofsaid heterojunction bipolar transistor.
 32. A semiconductor materialmade by the method of claim 18.